/usr/include/CGAL/Sqrt_extension/ is in libcgal-dev 4.2-5ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/CGAL/Sqrt_extension/Algebraic_extension_traits.h
/usr/include/CGAL/Sqrt_extension/Algebraic_structure_traits.h
/usr/include/CGAL/Sqrt_extension/Chinese_remainder_traits.h
/usr/include/CGAL/Sqrt_extension/Coercion_traits.h
/usr/include/CGAL/Sqrt_extension/Eigen_NumTraits.h
/usr/include/CGAL/Sqrt_extension/Fraction_traits.h
/usr/include/CGAL/Sqrt_extension/Get_arithmetic_kernel.h
/usr/include/CGAL/Sqrt_extension/Modular_traits.h
/usr/include/CGAL/Sqrt_extension/Real_embeddable_traits.h
/usr/include/CGAL/Sqrt_extension/Scalar_factor_traits.h
/usr/include/CGAL/Sqrt_extension/Sqrt_extension_type.h
/usr/include/CGAL/Sqrt_extension/Wang_traits.h
/usr/include/CGAL/Sqrt_extension/convert_to_bfi.h
/usr/include/CGAL/Sqrt_extension/io.h